The bss section is used for local common variable storage. You may allocate address space in the bss section, but you may not dictate data to load into it before your program executes. When your program starts running, all the contents of the bss section are zeroed bytes.
The .lcomm directive defines a symbol in the bss section; see Section 29.33: .lcomm.
The .comm directive may be used to declare a common symbol, which is another form of uninitialized symbol; see Section 29.9: Comm.
When assembling for a target that supports multiple sections, such as ELF or COFF, you may switch into the .bss section and define symbols as usual; see .section: Section. You may only assemble zero values into the section. Typically the section will only contain symbol definitions and .skip directives (see Section 29.55: Skip.).